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A Cervelo P4 came in recently with what can only be described as a poor attempt at repair. The top tube had cracked in an accident, someone had a go at repairing the damage by basically wrapping carbon fabric soaked with resin around the area, with no apparant compaction or pressure applied. This certainly doesn't meet the structural requirements and cosmetically looks quite ordinary as well. This is a $15000 bike!

I have seen quite a few "home made" repairs over the years, here is another one. This one is a Zipp 404 rim that was "repaired" at the braking surface with 5 min Epoxy! This stuff goes soft at about 70 deg C, so as soon as the brakes are used this epoxy goes soft and basically falls off. There are some basics that need to be adhered to, NO home fibreglass repair kits, NO 5 min epoxy, seek professional advice.

 

Here is a video showing the advantages of having Ultrasound scan equipment.

This fork showed no visual signs of any damage, however as you will see on the video, the ultrasound scan clearly shows that this fork should not be used.

The bike was involved in a crash where the handlebar swung around and hit the top tube of the frame. If you have had a similar accident we recommend that the fork steerer be thoroughly inspected.

 

 

 

Ultrasound scans are very useful for carbon composite inspection and is the primary method used in aerospace to determine the structural integrity of these parts.

Carbon composite structures are made up with a number of layers called plies, stacked on top of each other. Each ply needs to be bonded to the adjacent ply so it can transfer load. If this bond is compromised the structural integrity is significantly reduced.

Typical defects include:

Delaminations - where the plies are separated and can no longer transfer load. Very common after an impact or other failure.
Unbond - an unbond is when the plies or other fittings were not bonded properly during manufacture.
Porosity - dispersed air trapped in the resin during cure causing a reduction on mechanical properties. Aerospace standards are typically 2% max. porosity.
Void - a large air bubble trapped in the part.
Cracks - broken fibres and/or matrix.

Ultrasonic inspection is very sensitive to all these defects, it works by sending a high frequency sound wave into the part and then measuring what sound comes back. It takes very specialised equipment and knowledge.

Often highly loaded carbon composite parts such as aircraft components get 100% inspected by ultrasound methods during manufacture. This is usually conducted by automated inspection machines specific to the part and is very expensive. For aircraft repairs portable ultrasound systems are used to determine the damage prior to repair and most importantly to verify the structural integrity after repair.

Back to bikes!

So as carbon bikes become lighter and closer to the design limit it becomes very important to know that the structural material is made properly and capable of meeting the loads it was designed for. And just like aerospace, to repair high performance bikes you need to know what the damage is and that it has been repaired successfully.

 

 

We stress the importance of using resins with a high Tg (Glass transition temperature), which basically means the temperature that the resin can still provide structural performance. We only use aerospace approved repair resins that  have a Tg of over 120 deg C when cured properly, which is typical of the resin used during frame manufacture.

This is really important because if a lower Tg resin is used, the mechanical properties such as strength are reduced when the repair is at a higher temperature. This can happen if you have the bike in the car on a hot day and then go and ride. With a low Tg resin the repair can fail in this situation.

The thing is that the higher Tg resins need to be cured at elevated temperature to achieve these properties, so room temperature curing resins with lower Tg are generally not suitable for repair (unless you never ride in warm weather!)

So make sure when you get a carbon repair the correct resin and cure cycle is used to ensure correct performance, and avoid any repairs using general purpose resins such as those often used in boat building or other industrial applications.

This is a table listing the Tensile Strength and Modulus of most of the grades of carbon fibre raw material used wordwide. You can use it to see how Toray T700 difers from Graphil MR40. As you will see there are many grades available with a large range of properties, using different grades in different locations helps tune the ride of the humble bicycle. The other to note that isn't listed in this table however is the elongation properties, basically the starting point is about 2% and it goes down as the modulus increases. This stuff is not very elastic!